Drive mode sensors are utilized in a variety of sensing applications, such as, automatic transmission, and measuring drive mode. Such devices are typically utilized to convey the desired transmission gear position to the transmission control unit (TCU) (i.e. P-R-N-D-5-4 . . . ) in automotive application.
Various methods and systems are currently employed to accomplish this task, including mechanical and magnetic solutions. Magnetoresistive (MR) sensors, for example, are particularly beneficial for engine compartment (e.g., camshaft and crankshaft position sensing) and powertrain applications (e.g., transmission and transfer case gear speed sensing).
As vehicles become more complex, however, the controller requires more supportive information, such as, for example, information that instructs the TCU that the gearshift is between two positions (i.e. between reverse and neutral). Complex sensor-target (i.e., actuator) arrangements have also been devised to ensure that the vehicle will not start unless the sensor indicates that the vehicle is in park or neutral. Such devices also ensure that the TCU is not dependent simply upon one sensor signal.
U.S. Pat. No. 5,915,286, entitled “Safety Restraint Sensor System,” which issued to Bruce Figi, et al on Jun. 22, 2005 discloses an example of a safety restraint sensor application. U.S. Pat. No. 5,915,286, which is assigned to Honeywell International Inc. and is incorporated herein by reference, generally includes a safety restraint sensor system for detecting a latched position and/or an unlatched position of a latch and a buckle. A vane projects from the latch that is movably mounted with respect to the housing. A magnet and a Hall element are mounted with respect to the housing. When the buckle engages the latch, the vane and the buckle move within the magnetic field of the magnet. The magnetic field is distributed through the Hall element by way of the buckle and the vane. The Hall element sends a signal indicating the latched position to an appropriate receptor such as a safety restraint.
Other devices rely on magnetic angle attack configurations, such as, for example, a magnetic angle of attack sensor. Such a device typically includes a rotating vane that is sensitive to airflow direction mounted on a rotary shaft that is mounted on the housing. The housing can be supported on an aircraft, and includes a non-contact magnetic sensing assembly for sensing the rotation of the shaft relative to the housing.
The prior art suggests that an actuator consisting of an encoded magnet is a standard solution. The encoded magnet driven by the transmission manual valve has 4 magnetized ‘tracks’, each of which slides over a Hall sensor. The output of the TCU is a 4-bit code in which each position is indicated by a different code. Thus, only one sensor switches states with any magnet positional change and the system is not reliant on a single output.
The problem with Hall sensor/encoded magnet solutions, however, is two fold. First, desired accuracies are difficult to achieve (accuracy is ‘linear switch point accuracy’). Tolerances must be tightly held not only within the encoded magnetization pattern, but they also need to be held tightly regarding the magnet size, shape, the senor housing size and shape, and the placement of the Hall effect (HE) sensor. Second, the cost of such a solution is high due to the difficult nature of holding tolerances and the large size of the magnet required. A point to note is that mechanical solutions are not considered as the designs typically wear; reliability is considered superior in the electronic/magnetic solutions.
Variable reluctance (VR) sensors offer low cost, do not require power, and are easy to connect to the ECU (electronic control unit), but the amplitude of its output signal is proportional to rotational speed, thus making it inaccurate at low speeds and/or high noise. While, the differential magnetic comparator (DMC) configuration, which uses two closely spaced HE sensors and compares the difference in the signals allows for detecting zero speed (which cannot be detected using single Hall sensors) are relatively more costly.
The magnetoresistive (MR) sensors use a sophisticated mirrored target, a ferromagnetic disc with two tracks. The resulting signal, which is a large signal, can be used to accurately detect position and speed (including zero speed) with repeatability, and good tolerance for mechanical variation. However, mirrored targets are relatively more expensive than gear or vane sensor/target configurations.
Hence, there is an apparent need for an inexpensive and easily manufactured drive mode sensor system to address the aforementioned problems effectively.